1. Field of the Invention
The present invention relates to a base station apparatus that carries out a radio communication by switching between transmission and reception every predetermined unit time using a plurality of carriers and a transmission branch selection method.
2. Description of the Related Art
In a mobile communication system, it is proposed that a base station apparatus should select a transmission branch when sending a signal to a mobile station apparatus in order to alleviate the load of signal processing on the mobile station apparatus.
Suppose a group of signals transmitted or received for a unit time of switching between transmission and reception is called xe2x80x9cburstxe2x80x9d here. The base station apparatus alternately performs 1-burst (downlink) transmission and 1-burst (uplink) reception.
A conventional base station apparatus and transmission branch selection method are explained below using FIG. 1 and FIG. 2. FIG. 1 is a block diagram showing a configuration of a reception system in a conventional base station apparatus and FIG. 2 is a block diagram showing a configuration of a transmission system in the conventional base station apparatus.
In FIG. 1 and FIG. 2, suppose the number of subcarriers is 4 and the number of branches is 2.
In FIG. 1, signals received from branch 1 and branch 2 (reception signals) are converted to digital signals by A/D converters 101 and 102 and FFT-processed by Fast Fourier Transform (hereinafter referred to as xe2x80x9cFFTxe2x80x9d) circuits 103 and 104.
The reception levels of the FFT-processed reception signals are detected by level detectors 105 to 112 for their respective subcarriers. The detected reception levels are determined on a subcarrier basis by large/small comparison sections 113 to 116 as to which has a higher reception level, the signal received from branch 1 or the signal received from branch 2.
The determination results are used as control signals 1 to 4 corresponding to their respective subcarriers for switching by switches 117 to 120 and for transmission branch selection in a transmission system which will be described later.
The FFT-processed reception signals are also input to switches 117 to 120 and the reception signal with a higher reception level between the reception signal from branch 1 and the reception signal from branch 2 is output for each subcarrier from switches 117 to 120 through switchover control by control signals 1 to 4.
The output signals of switches 117 to 120 are subjected to delay detection processing by delay detectors 121 to 124, determined by determination circuits 125 to 128 and output to parallel-serial (hereinafter referred to as xe2x80x9cP/Sxe2x80x9d ) converter 129. P/S converter 129 converts a plurality of parallel signals from determination circuits 125 to 128 to a serial signal and obtains a demodulated signal.
On the other hand, in FIG. 2, the modulated data are converted by serial-parallel (hereinafter referred to as xe2x80x9cS/Pxe2x80x9d ) converter 201 from a serial signal to a plurality of parallel signals. The plurality of parallel signals from S/P converter 201 is each mapped by mapping circuits 202 to 205.
For the signals mapped by mapping circuits 202 to 205, either branch 1 or branch 2 is selected as their output destination by switches 206 to 209 using control signals 1 to 4 described with reference to FIG. 1.
Inverse Fast Fourier Transform (hereinafter referred to as xe2x80x9cIFFTxe2x80x9d ) circuits 210 and 211 perform IFFT processing on signals to be transmitted from branch 1 and branch 2, respectively. The IFFT-processed signals by IFFT circuits 210 and 211 are converted from digital to analog signals by D/A converters 212 and 213.
As described above, the conventional base station apparatus detects reception levels of all subcarriers of reception signals at all branches, carries out reception diversity to determine which signal has the highest reception level received from different branches for each subcarrier and select a branch from which to receive signals based on this determination result, and transmission diversity to select a branch from which to transmit signals.
However, if there is a certain time interval after an uplink burst is received until a downlink burst is transmitted, that is, in the case of an intermittent communication condition, the radio propagation environment characteristic may change in the above interval in the conventional base station apparatus. Therefore, if a branch is selected based on the reception level of the immediately preceding uplink burst to transmit each subcarrier of the next downlink burst, accurate branch selection is not made.
Moreover, if a setting is made in such a way that branches can also be switched during 1-burst transmission, bursts become discontinuous on the mobile station apparatus side causing the error rate characteristic to deteriorate.
The present invention has been implemented taking account of the points described above and it is an objective of the present invention to provide a base station apparatus and transmission branch selection method carrying out accurate transmission branch selection even in an intermittent communication condition with a certain time interval after an uplink burst is received until a downlink burst is transmitted.
This objective is achieved by the base station apparatus estimating the reception levels of the next uplink bursts to be received from the reception levels of a plurality of uplink bursts and selecting a branch to transmit each subcarrier of the next downlink burst to be transmitted based on this estimated value.